fix broken link to OpenShift installation description (#274)

The current link in this document points to openshift-install.md which was removed in pull #238 

commit 366c8b2
Author: Rajdeep Das <rajdeep.das@mongodb.com>
Date:   Mon Jan 30 14:15:48 2023 +0100

The correct document appears to be docs/openshift-marketplace.md

Signed-off-by: Jens Freimann <github@freimann.org>

Author: jensfr

README.md

@@ -70,7 +70,7 @@ CRDs are defined cluster-wide, so to install them, you must have Cluster-level a
 
 #### Operator Installation
 
-> In order to install the Operator in OpenShift, please follow [these](openshift-install.md) instructions instead.
+> In order to install the Operator in OpenShift, please follow [these](docs/openshift-marketplace.md) instructions instead.
 
 To install the Operator using yaml files, you may apply the config directly from github;